Experiments for air-water flow with and without added foamers were performed in a 50 mm diameter 12 m long vertical pipe at ambient pressure. It was observed that adding foamers to water will lead to a lower pressure drop at superficial gas velocities below the transition limit from annular flow to churn annular flow (which is around 15 m/s) and at superficial liquid velocities between 0.5 and 2 cm/s. Visualisation of the flow with a high speed camera indicates that the decrease in the pressure drop is due to the more regular nature of the flow when the water is foaming: churning of the flow is suppressed by the foam. This is confirmed by the decrease of the pressure oscillations in the presence of foamers. These experiments give insight into why and how liquid loading in gas wells is prevented by the addition of foamers.

1 INTRODUCTION

In a gas well, both liquids – in the form of water and gas condensate – and gas are produced. If the reservoir pressure is high, the gas velocity in the well tubing is sufficient to drag the liquids to the surface. However, near the end of field life when the reservoir pressure has depleted, the gas velocity becomes too low to transport the liquids through the well. The minimum gas velocity required to lift the liquids is called the criticalvelocity. When the gas velocity becomes lower than the critical velocity, liquid will start accumulating at the bottom of the well. This will generate a large hydrostatic pressure in the well, which severely limits the gas production. This process is called liquid loading (1). Most foamers used in gas wells foam only the water (2), but there also exist some condensate foamers (3). In this work, only a water-based foamer has been considered.